The present invention relates to a DS-CDMA multiple user serial interference canceler and, more particularly to a method of transmitting an interference replica signal.
Conventionally a multiple user serial interference canceler has been proposed for reducing interference within a cell of a DS-CDMA. In the above multiple user serial interference canceler, an interference replica signal of a user (other than a user k) (1.ltoreq.k.ltoreq.K) that has been demodulated previous to demodulation of the user k is generated. The generated interference replica signal is subtracted from the received signal and this process is repeated N times (N: natural number .gtoreq.1, representing the number of stages) so that influence owing to other user's interference can be reduced.
In a usual manner to transmit the interference replica signal of other user, a bus line that can be shared and accessed by all panels is provided so that the interference replica signal from each panel is notified and synthesized with that of other panel. FIG. 3 is an example of a construction of a conventional interference canceler for the interference replica transmission through the common bus line. A reference numeral 31 is a received signal, 32 is a common bus line for interference replica transmission, 33 is an interference canceler unit (including decoder), 34 is a delay memory for matching timing between the received signal and the interference replica signal, 35 is a subtractor for subtracting the interference replica signal from the received signal, 36 is a processor for synthesizing the interference replica signal of other user, and 37 is a decoder. The common bus line 32 is connected to all users at all stages so that the interference replica signal generated by each user at each stage is transmitted to other users. Each user synthesizes the transmitted interference replica in the processor 36, which is subjected to interference cancellation through subtraction from the received signal in the subtractor 35. The resultant signal is transmitted to the interference canceler unit (hereinafter referred to as an ICU) 33. The decoder 37 at the last stage (third stage in FIG. 3) outputs the final decoding result. The parameter defining the transmission rate of the signal running through the common bus line 32 is spreading chip rate, n sampling times, K users and N stages. The transmission rate obtained by (spreading chip rate .times.n sampling times .times.K user .times.N stage) is required.
In a conventional method, the common bus line is used to send the interference replica signal of each user to all users other than the present user. In the above method, a signal at a transmission rate K.times.N times higher than that of the interference replica signal equivalent to one user runs through the common bus line. For example, assuming that the transmission rate of the interference replica signal is 120 Mbps, the number of users is 16 and the number of stages is 3, respectively, the transmission rate required for the common bus line becomes 5.76 Gbps. Therefore it is practically difficult to design the circuit conforming to the above condition.
In the above-described prior art, as generation of the interference replica signal proceeds to the latter phase (the number of ICU#1-K increases), the delay is accumulated. As a result, the received signal delay holder memory D for delaying and holding the received signal has to be increased. The whole system, thus, becomes unnecessarily large.